use old countertop as template to outline new
cut melamine boards to exact size of old counter
use fiberglass or masonite strips to create curves
Build the Countertop Mold
The countertop mold is made of 3/4" dual-sided laminate melamine (laminate veneer is on both sides). It’s relatively inexpensive and up to the task.
Use the old countertop as a template to outline the outside edges and corners on the bottom piece of melamine (Image 1).
Using a table saw, circular saw and jig saw, cut the melamine boards to the exact size of the old countertop (Image 2).
Cut 2" melamine strips for the sides and ends (four altogether) of the mold.
Screw the melamine strips and bottom piece together with 1-3/4" countersunk screws to create a box (Image 3) for the concrete mold. Smooth out any rough edges with an orbital sander.
Use fiberglass or Masonite strips to create curves for the corners (Image 4).
The mold has to be watertight to keep the water in the cement from causing discolorations, blemishes and dry areas in the countertop. To ensure that it’s watertight, all seams and joints have to be caulked with silicone. But silicone can also discolor concrete, so it's important to smooth out the silicone very neatly and evenly to form an unobtrusive edge in the seams.
Before caulking, tape all interior planes of the mold (with painter's tape) on either side of all seams and joints, leaving just enough room for a bead of caulk to be applied. The tape prevents the silicon from affecting other parts of the countertop.
Caulk all seams, edges and joints where the melamine pieces come together.
When it’s all caulked go back over the bead with a dab of denatured alcohol on a finger, wiping the silicone smooth along all edges.
After the silicone dries, carefully peal the painter’s tape, pulling it into itself to pull it off as well as any stray silicone.
Affix any items for "relief" or "inlay" effects (these are objects that will stay in place once the concrete is set/cured) with spray adhesive to the base of the form. For this project, DIY Host Paul Ryan used a small leaf (Image 5) as the relief effect.
To make sure the concrete will release from the mold easily, douse a rag with kerosene-based release oil and cover every surface of the mold completely.
Fill all screwhead holes with colored modeling clay, which keeps cement out of the screw heads and makes for easier disassembly of the mold.
Using bolt or wire cutters, cut the "hog fence" wire mesh (Image 6) for reinforcement inside the mold, leaving about 1-1/2” around all edges. You can suspend the mesh by securing screws into the outside surface of the mold to which you secure wire hooks to “hang” the mesh inside the mold in the middle of the cement. Or as shown below, just after screeding the concrete (see Step 4), place the mesh evenly into the concrete and push it down, suspending it in the middle of the concrete.